a. Field of the Invention
The present invention relates to a system for measuring the vibration frequency of a vibrating object, and in particular it pertains to a method for measuring the vibration frequency of a vibrating object, such as a string of a piano, which develops harmonic vibrations, and also it concerns a method for adjusting the vibration frequency of the vibrating object.
B. Description of the Prior Art
A number of vibrating objects (strings) are used in a musical instrument such as a piano. By causing vibration of such vibrating object, there has been obtained the production of a sound having a frequency corresponding to the fundamental frequency of the vibrating object. In such a case, the vibration frequency of the vibrating object tends to undergo a side variation depending on the factors such as the trifle dimensional errors arising at the time of the manufacture of the vibrating object, the unevenness of the quality of the material with which the vibrating object is made, and the conditions in which the vibrating object is assembled into a musical instrument. Accordingly, in a musical instrument wherein a precision vibration frequency is required, there is carried out adjustment of the vibration frequency of the vibrating object upon completion of the assemblage of the whole musical instrument, and also periodically thereafter. In conducting such adjustment of the vibration frequency of the vibrating object, there has been employed in the past a tuning method which entirely depends on the acoustic sense of a well-trained tuning expert. Recently, however, there is relied upon the method consisting of an electrical detection of the vibration frequency of the vibrating object and then of carrying out the tuning in such a way that the detected value is adjusted so as to agree with a reference value. This latter method will hereunder be described in further detail by referring to FIG. 1. By the use of a sensor 1 such as a microphone, the vibration frequency of a vibrating object not shown is converted to an electric signal. This electric signal is derived via a band-pass filter 2 having a middle point of a pass band set at the frequency to which the vibration frequency of the vibrating object is to be tuned in and also via an amplifier 3. The tuning operation is carried out by a tuner by adjusting, for example, the tuning pin of a piano in such a way that the output of the amplifier 3 will present a peak level of amplitude.
It should be noted that such a vibrating object as the strings of a piano involves harmonic vibrations in addition to the fundamental vibration frequency. Accordingly, the output electric signal derived from the sensor 1 contains a fundamental frequency component f and its harmonic components nf (n=2, 3, . . . ). In such a vibrating object such as the string of a piano, it should be noted that, in case its fundamental vibration frequency is low (meaning it is in the bass region), the spectrum of the harmonics is as shown in FIG. 2. That is, the amplitude level of the harmonic component is high as distinctly contrasted by an extremely low amplitude level of the fundamental frequency component. Moreover, the microphone assigned to detect the vibration of the vibrating object generally presents a poor sensitivity to the lower frequencies, and besides, there also arise problems such as noises. As such, in conducting the tuning of a piano string of the lower pitch range, it is difficult to carry out a direct detection of the fundamental vibration frequency component. The tuning of such a string requires to be done by detecting the harmonic components. In case it is intended to tune the fundamental vibration frequency into for example 27.5 Hz, there has been employed a band-pass filter whose middle point of the band is set at 110 Hz which is the frequency of the quadruple harmonic, to carry out the tunning in such a way that the output of this band-pass filter will present a peak level. However, in the low pitch range of sound, the difference in frequency between the respective adjacent harmonic sounds is small and moreover, there is a limit in the improvement of the selectively of filters. Therefore, any two adjacent harmonic signal components undesirably jointly pass through the band-pass filter 2. Thus, it becomes impossible to identify which one of them is in fact the aimed harmonic component to be measured. In an extreme instance, the tuner may erroneously tune the vibration frequency of the vibrating object to a harmonic (n+1)f or (n-1)f which is located just adjacent to the aimed harmonic partial nf. For example, let us consider the instance wherein the tuner intends to tune the vibration frequency of a vibrating object having the fundamental vibration frequency of sound of 22.5 Hz prior to tuning so as to have the fundamental vibration frequency of 27.5 Hz. In order to do this, let us assume that the tuner employs a band-pass filter 2 having the filter characteristic A whose middle point is set at 110 Hz frequency of the quadruple harmonic. Whereupon, a signal of 90 Hz which is the quadruple harmonic of the fundamental frequency 22.5 Hz prior to tuning and a signal of 112.5 Hz which is the quintuple harmonic of this 22.5 Hz harmonic are allowed to pass jointly through the filter owing to the fact that these two frequencies are included in the width of the pass band of this band-pass filter. Accordingly, in each of the tuning operations that the tuner (tuning engineer) turns the tuning pin of, for example, a piano clockwise and counterclockwise, the output signal derived will only increase, and it becomes altogether impossible for the tuner to identify which one of them is the quadruple harmonic sound signal which is aimed. Such phenomenon will be prominent in an instance wherein the fundamental vibration frequency of the vibrating object is departed toward the lower level side than that which is aimed.
Furthermore, in case the vibration frequency of the vibrating object is departed so far as to be on the outside of the pass band of the band-pass filter 2, it is utterly impossible to obtain an output. Moreover, it is impossible also to known in which direction the vibration signal is departed. In case it is intended to adjust the vibration frequency of a string in particular, it often happens that the vibration frequency of the string prior to tuning is founnd to depart considerably from the vibration frequency which is aimed. Thus, the above-discussed problem will take place prominently. Also, in case such conventional measuring system as stated above is relied upon, there may arise instances wherein no output signal is derived. Thus, this known measuring system is not suitable for use as the vibration frequency measuring system of an automatic tuning device which is designed to use a signal which is formed by converting the vibration of the vibrating object to an electric signal to thereby automatically tune the naked vibration frequency into the vibration frequency which is aimed. Especially, in the treble (high frequency) range, the amount of departure of the vibration frequency becomes great, and therefore the above-discussed known measuring device will be found difficult to operate.